entity-framework - Entity Framework 核心 : Include many-to-many related objects in WebAPI

Question

我不太熟悉 .NET 框架，但决定试用 ASP.NET Core 和 EF Core。我想制作一个非常简单的 Web API 后端，但我在处理多对多关系时遇到了问题。

我知道我需要为这两个实体创建一个关系表，如本文中的示例所示:How to create a many to many relationship with latest nightly builds of EF Core?

我还让我的模型构建器按照此处描述的方式创建关系 http://ef.readthedocs.io/en/latest/modeling/relationships.html#many-to-many

我的代码是这样的:

在数据库上下文中:

protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);

        builder.Entity<MovieActor>().HasKey(x => new { x.MovieId, x.ActorId});

        builder.Entity<MovieActor>()
            .HasOne(m => m.Movie)
            .WithMany(ma => ma.MovieActors)
            .HasForeignKey(m => m.MovieId);

        builder.Entity<MovieActor>()
            .HasOne(m => m.Actor)
            .WithMany(ma => ma.MovieActors)
            .HasForeignKey(a => a.ActorId);

    }

    public DbSet<Movie> Movies { get; set; }

    public DbSet<Actor> Actors { get; set; }


}

模型:

namespace Test.Models
{
    public class Movie
    {
        public int MovieId { get; set; }
        public string Title { get; set; }
        public DateTime ReleaseDate { get; set; }
        public string Code { get; set; }
        public string Path { get; set; }

        public ICollection<MovieActor> MovieActors { get; set; }
    }

    public class Actor
    {
        public int ActorId { get; set; }
        public string Name { get; set; }
        public ICollection<MovieActor> MovieActors { get; set; }
    }

    public class MovieActor
    {
        public int MovieId { get; set; }
        public Movie Movie { get; set; }

        public int ActorId { get; set; }
        public Actor Actor { get; set; }
    }
}

这似乎在大多数情况下都有效。我可以创建新的 MovieActor 对象并将它们添加到数据库中。一件看起来很奇怪的事情是 Movie 和 Actor 模型现在有一个 MovieActors 属性，但它总是 null，尽管连接表有条目。

我想不通的是如何在我的 WebAPI Controller 中检索这些相关对象。我的 Linq 技能很弱，所以也许我只需要复习一下，但我不知道从哪里开始。当我通过 ID 获取电影或获取所有电影的列表时，我希望结果在对象的数组中包含任何相关的 Actors。在另一个方向上也是如此，当获得一个 Actor 时，我想获得一系列他们的电影。

这是我尝试利用这种关系的 Controller 方法之一:

        // POST: api/Movie
    [HttpPost]
    public IActionResult PostMovie([FromBody] MovieBundle Moviebundle)
    {


        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        var _movie = _context.Movies.FirstOrDefault(m => m.Code == Moviebundle.Movie.Code);
        if(_movie == null)
        {
            //Doesn't exist! add a new one
            _movie = Moviebundle.Movie;
            _context.Movies.Add(_movie);
            _context.SaveChanges();

            foreach (Actor actor in Moviebundle.actors)
            {
                //Try to look up by name
                Actor _actor = _context.Actors.FirstOrDefault(a => a.Name == actor.Name);

                //If they don't exist, add them to the DB 
                if (_actor == null)
                {
                    _context.Actors.Add(actor);
                    _context.SaveChanges();
                    _actor = actor;
                }

                //This is what I'm using to create the M-t-M relationship
                var link = new MovieActor { Actor = _actor, Movie = _movie };
                _context.MovieActor.Add(link); //This line gives an error


            }


        } else
        {
            return new StatusCodeResult(StatusCodes.Status409Conflict);
        }




        try
        {
            _context.SaveChanges();
        }
        catch (DbUpdateException)
        {
            if (MovieExists(_movie.MovieID))
            {
                return new StatusCodeResult(StatusCodes.Status409Conflict);
            }
            else
            {
                throw;
            }
        }

        return CreatedAtAction("GetMovie", new { id = _movie.MovieID }, _movie);
    }

Answer 1

您必须如下所示更改它。

模型:

namespace Test.Models
{
    public class Movie
    {
        public int MovieId { get; set; }
        public string Title { get; set; }
        public DateTime ReleaseDate { get; set; }
        public string Code { get; set; }
        public string Path { get; set; }

        public ICollection<MovieActor> MovieActors { get; set; }
    }

    public class Actor
    {
        public int ActorId { get; set; }
        public string Name { get; set; }
        public ICollection<MovieActor> MovieActors { get; set; }
    }

    public class MovieActor
    {
        public int MovieId { get; set; }
        public Movie Movie { get; set; }

        public int ActorId { get; set; }
        public Actor Actor { get; set; }
    }
}

上下文:

protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);

        builder.Entity<MovieActor>().HasKey(x => new { x.MovieId, x.ActorId});

        builder.Entity<MovieActor>()
            .HasOne(m => m.Movie)
            .WithMany(ma => ma.MovieActors)
            .HasForeignKey(m => m.MovieId);

        builder.Entity<MovieActor>()
            .HasOne(m => m.Actor)
            .WithMany(ma => ma.MovieActors)
            .HasForeignKey(a => a.ActorId);

    }

    public DbSet<Movie> Movies { get; set; }

    public DbSet<Actor> Actors { get; set; }

    public DbSet<MovieActor> MovieActors { get; set; }

 }

您可以导航，如下所示:

var actor = context.Actors
                   .Include(r => r.MovieActors)
                   .FirstOrDefault(a=>a.ActorId = 1)

var movies = actor.MovieActors.Select(c => c.Movie);

如何添加记录:

var actor = new Actor { ... };
context.Actors.Add(actor);

var movie = new Movie { ... };
context.Movies.Add(movie);

var movieActor = new MovieActor { Actor = actor , Movie = movie };
context.MovieActors.Add(movieActor);

context.SaveChanges();